Meet Iowa's big-tree hunter

Mark Rouw has been documenting biggest trees since the 1970s

Iowa's champion black walnut tree, pictured last Monday, is on the Pentacrest near Macbride Hall in Iowa City. In 2016,
Iowa's champion black walnut tree, pictured last Monday, is on the Pentacrest near Macbride Hall in Iowa City. In 2016, the tree measured 16 feet and 11.5 inches in circumference and 92 feet tall with a canopy spread of 100 feet. (Cliff Jette/The Gazette)

Trees that survive storms, disease, pests and development, rising through several human generations to become the biggest of their kinds, have a way of putting our lives into perspective.

Mark Rouw has been finding and documenting the state’s largest trees for more than 40 years to help Iowans learn more about the natural environment and to provide some protection for the quiet giants.

“I call him the ‘Rain Man’ of trees,” University of Iowa Arborist Andy Dahl said of Rouw, referring to Rouw’s near-perfect recall of the locations of big trees throughout the state. “It’s just amazing what he does.”

Rouw, 61, of Des Moines, maintains Iowa’s list of state champion trees and their runners-up as an unpaid volunteer, updating an ever-changing spreadsheet of the state’s largest trees of species from Balsam Fir to Rusty Blackhaw. He drives out to river bottoms and uplands to measure newly-discovered big trees and make sure state champions still are standing.

“It’s a little bit like birders, who are searching for a new bird on their life list,” Rouw said. “If I find a big tree, the next step is to find one that is even bigger.”

Protecting big trees

The Iowa Department of Natural Resources started the Iowa Big Tree Program in 1978, inspired by the national Big Tree Program, launched in 1940 by American Forestry Association, a nonprofit conservation group now called American Forests.

“So many of the nation’s largest trees were being cut, mostly for lumber,” Rouw said of the genesis of the national program. “This idea came up to hopefully protect some of the biggest trees. If they were identified as being the largest one, hopefully they wouldn’t be cut. It worked to some degree.”


The national program has a searchable database of more than 700 national champions.

The Iowa DNR used to dispatch district foresters to verify measurements of trees nominated for the state big tree program, but that funding is gone. Now, when someone submits a nomination form to the Iowa DNR, the agency sends the paperwork to Rouw, designated as a field representative, he said. Rouw has got a day job as an animal specialist at the Science Center of Iowa, but big trees have been a passion since he was a child.

“When I first got interested in trees, it was 1971,” Rouw said. A Des Moines neighbor would drive Rouw out to his farm on weekends, pointing out tree varieties as they passed in the car. Rouw learned to recognize tree species from a distance by their form and texture. Later he started hunting for big trees.

When he was 15, Rouw found an Eastern cottonwood with a trunk circumference of about 20 feet — a benchmark he still uses to denote the biggest cottonwoods.

The river-bottom dwellers with craggy bark, trembling leaves and seeds released in tangles of cottony fiber are the largest tree variety in Iowa, Rouw said. The state champion cottonwood, in Crawford County, was last measured at 31.6 feet in trunk circumference, 81 feet tall with a 93.5-foot canopy spread.

Tree hunting

In both the national and state big tree programs, trees are scored on a point system that tries to standardize life-forms that grow in all shapes and sizes. Every inch of a trunk’s circumference at 4.5 feet above ground gets a point. Every foot of height gets a point and every foot of canopy spread gets a quarter point, Rouw explained.

“It favors big trunks,” he said. “I’d like to see each measurement as equal.”

A tree’s height is measured by triangulation, done when the measurer stands 100 feet away from the base of the tree. Looking through a clinometer, or small device somewhat like a compass, the measurer raises the device by tilting his head to place the sighting line so is aligns with the tallest branch of the tree. That distance is added to the distance from eye level to the base of the tree to get the total height.


But you have to be careful to pick the tallest branch, not just one that appears the tallest because it’s closer to you, Rouw warned. Sighting off a long side branch can falsely add to the height measurement. Rouw now uses a laser rangefinger that is accurate within inches.

Top counties by big tree champions and runners-up

Top Counties for Big Trees in Iowa
Des Moines37
Lee 19
Henry 8

Rouw has documented state champion trees in Iowa, Nebraska, Kansas and Colorado. He’s found national champs in Iowa, Colorado, South Dakota, Arizona and Texas.

One of his most significant finds was a big blue ash in Lee County, which is far west for this species. That tree is approaching national champion size — if it can ward off the emerald ash borer, a wood-boring beetle found in nearly two-thirds of Iowa counties, Rouw said.

Eastern Iowa giants

Eastern Iowa is home to many of Iowa’s state champion trees, from a white pine in Fayette County that is the state’s tallest tree at more than 148 feet to a European larch that grows out of a hillside in Dubuque’s Linwood Cemetery.

College campuses claim a handful of these giants, from a ginkgo on the Cornell College campus in Mount Vernon to a black walnut that shades a large swath of the University of Iowa Pentacrest in Iowa City.

“I’ve got a photo of that walnut from 1907, when Macbride was being built,” Dahl, the UI arborist, said of the limestone building the tree guards.

The walnut was struck by lightning in the early 1980s, which spurred the UI to install a lightning protection system in 1982, Dahl said. But the tree grew past the copper terminal designed to conduct electricity to the ground without harming the tree. A new system was put in place in 2013.

“We want to do everything we can to save a historic tree like that,” Dahl said.

Some state champions are in private yards, outliving the residents who planted them and dwarfing the houses they shade.


One of those is a nearly 70-foot-tall butternut in the Lisbon yard of Alex Carls, 28, a Springville art teacher. The tree was a plus when she bought the house last year.

“I thought it was cool,” she said.

Carls is glad to share tree maintenance with Sara Ellison, 68, who has lived next door 41 years. The biggest task is cleaning up the butternuts that fall from midsummer through the fall. One year, Ellison collected 32 10-gallon garbage bags full of the nuts without many modern-day uses.

“We have very well-fed squirrels, I can tell you,” Ellison said.

Being the caretaker of a century-old state champion tree is an honor, she said. “It’s a wonderful tree.”

The Iowa Department of Natural Resources relies on volunteers to check trees nominated for the state Big Tree Program. Since there’s no state funding for this program, volunteers like Mark Rouw, of Des Moines, pay for gas and other expenses connected with documenting new trees and updating measurements for state champions.

How to donate to Big Trees of Iowa

Iowans can donate to this program through a form on the Iowa DNR Forestry bureau’s website at

There, you can also find a form for nominating big trees and instructions for how to measure them.

Iowans may contact Rouw directly at or (515) 271-9315.

l Comments: (319) 339-3157;

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